轧后加速冷却开始温度对高应变管线钢组织和力学性能的影响

Effect of Temperature at Start of Accelerated Cooling Subsequent to Rolling on Microstructure and Mechanical Properties of High-Strain Pipeline Steel

高应变管线钢热轧后先空冷,随后再加速冷却。采用光学显微镜、扫描电子显微镜、电子背散射衍射及拉伸试验研究了热轧后加速冷却的开始温度对高应变管线钢显微组织和力学性能的影响。结果表明:随着加速冷却开始温度的降低,管线钢中铁素体含量增加,残留奥氏体转变为马氏体而不是粒状贝氏体,硬相贝氏体或马氏体的硬度从279 HV0.02提高至522.2 HV0.02;热轧后从710、670和630℃加速冷却的管线钢强度分别达到X60、X56和X52级,最大力总延伸率较高,约为13%;热轧后从不同温度加速冷却的管线钢的应变硬化指数n值均大于0.1,但随着应变范围的变化,n值发生明显变化,选择合理的应变范围对于根据n值评价高应变管线钢的应变硬化效应至关重要。

High-strain pipeline steel is cooled in air at first and then is cooled at accelerated rate after hot-rolling.The effect of temperature from which the accelerated cooling started on microstructure and mechanical properties of a high-strain pipeline steel was investigated by the aid of optical microscope,scanning electron microscope,electron back scattered diffraction and tensile test.The results showed that(a)with the decrease in temperature at the start of accelerated cooling,ferrite content in the pipeline steel increased,retained austenite transformed into martensite rather than granular bainite,and hardness of the hard phase such as bainite or martensite increased from 279 to 522.2 HV0.02;(b)the pipeline steel accelerated-cooled from 710,670 and 630℃after hot-rolling exhibited X60,X56 and X52 grades of strength,respectively,and the total extension at maximum force as relatively high as about 13%;and(c)strain-hardening exponent n values of the pipeline steel accelerated-cooled from different temperatures after hot-rolling were all greater than 0.1,however,as the strain range changed,n value varied significantly,and selection of a reasonable strain range was very important to evaluation of the strain-hardening effect of the high-strain pipeline steel according to n value.